1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus.
2. Description of the Related Art
Conventionally, in some image forming apparatuses, such as copying machines, printers, facsimile apparatuses, and multi-function apparatuses composed of combinations of these, an image forming apparatus main body is provided with a sheet processing apparatus for performing a processing, such as alignment or binding, on sheets delivered from the image forming apparatus main body. In an example of such the sheet processing apparatus, the delivered sheets are stacked and aligned on a process tray, and then a processing, such as binding, is performed on the sheets (i.e., sheet bundle).
FIG. 12 is a diagram showing an example of such the conventional sheet processing apparatus. In a sheet processing apparatus 100A, sheets that have undergone image formation in the image forming apparatus main body are temporarily stacked on a process tray 138, on which a processing, such as alignment or binding, is performed on the sheet bundle. When the processing is completed, the sheet bundle is delivered onto a stack tray 137 with an inclined sheet bundle surface by sheet bundle delivery rollers 130.
In the sheet processing apparatus 100A, a length in the sheet transport direction (hereinafter, simply referred to as “length”) of the process tray 138, that is, a distance from a trailing end regulating member 3 for regulating trailing ends of the sheets to the sheet-bundle discharge rollers 130, is 180 mm or less. Thus, when sheets of a large size, such as A3 or LDR, are to be processed, while the sheet trailing ends are stacked on the process tray 138, leading ends of these are stacked on the stack tray (or on the already stacked sheets) for alignment (see Japanese Patent Application Laid-open No. H11-334975).
This construction is effective in achieving space saving for the apparatus as a whole. Further, it is possible to transport sheet bundles of half sizes, such as A4 and B5, by the sheet-bundle discharge rollers 130 alone.
FIG. 13 is a diagram showing a conventional sheet processing apparatus of another construction. In this sheet processing apparatus, a length of a process tray 209, that is, a distance from a trailing end regulating member 210 to a sheet-bundle discharge roller 208, is larger as compared with that of the process tray 138 of the sheet processing apparatus 100 shown in FIG. 12. Thus, even sheets of large sizes, such as A3 and LDR, can be aligned, with their entire surfaces placed on the process tray.
With this construction, even in a case in which large size sheets are aligned, the sheets are not exposed to the exterior of a sheet processing apparatus main body F, so there is no fear of a user erroneously extracting the sheets before the processing is over.
However, in the sheet processing apparatus F, the distance from the trailing end regulating member 210 to the sheet bundle discharge roller 208 is large, so it is impossible to transport the sheet bundles of half sizes, such as A4 and B5, by the sheet bundle discharge rollers 130 alone. Thus, a sheet bundle thrust member 219 is separately provided. When transporting the half-size sheet bundle, the sheet bundle is delivered by transporting it to the sheet bundle discharge roller 208 by the sheet bundle thrust member 219 (see Japanese Patent Application Laid-open No. 2000-075573).
In conventional sheet processing apparatuses, there are cases in which, for example, sheets with surfaces of a small coefficient of friction with color images transferred thereto are to be aligned. In this case, when the sheet bundle is delivered by the sheet bundle discharge rollers 130 alone as in the case of FIG. 12, upper sheets Pa and lower sheets Pb of a sheet bundle PA can be transported by the sheet bundle discharge rollers 130 as shown in FIG. 14. However, sheets Pc in a central portion of the sheet bundle PA slip, so sheet bundle transport cannot be effected properly.
In a case in which, as shown in FIG. 13, the sheet bundle thrust member 219 is arranged, the sheet bundle can be delivered reliably. However, even when the sheet bundle can be transported by the sheet bundle discharge roller 208 alone, it is necessary to operate the trailing end thrust member 219.
Further, even in, for example, a double binding mode, in which a sheet bundle is bound by stapling at two positions and in which the sheets Pc in the central portion of the sheet bundle PA do not slip, so the trailing end thrust member 219 is not necessary, the trailing end thrust member 219 is operated. Thus, there are involved problems, such as large power consumption, generation of noise, and impairing of durability of the trailing end thrust member.
Further, in order that the sheet bundle may be transported by the trailing end thrust member 219 even when the sheet bundle contains a large number of sheets (e.g., 100 sheets), a large motor is employed. Here, when a large motor is thus employed, an impact when the trailing end thrust member 219 abuts the sheet bundle is large if, in particular, the number of sheets in the sheet bundle is small, so, to mitigate the impact, the trailing end thrust member 219 is moved at low acceleration (i.e., at low speed).
However, when the trailing end thrust member 219 is thus moved at low acceleration (i.e., at low speed), a requisite processing time is rather long. That is, when a large motor is employed, not only is the size of the apparatus increased but also the requisite processing time increases when the trailing end thrust member 219 is moved at low acceleration (i.e., at low speed) in order to mitigate the impact when the trailing end thrust member 219 abuts the sheet bundle, thereby resulting in a deterioration in productivity.